1. Field of the Invention
The present invention relates to cameras with electronic flash capability and, in particular, to a system for improving the quality of direct flash illumination, by automatically coupling flash spread to subject distance.
2. Description of the Prior Art
In the field of photography, there are numerous types of cameras available that have dedicated flash systems. Many of these cameras include a built-in flash unit that is an integral part of the camera. The field of illumination of such flash units is often designed to be less than the angle-of-view of the objective lens of the camera in order to maximize the flash range, that is, to obtain maximum subject distance for an acceptable flash exposure. However, when the angle-of-illumination is less than the angle-of-view of the objective lens, the center of the image field receives a disproportionately high percentage of the total illumination. As a result, the flash illumination, especially at short camera-to-subject distances, is harsh and displeasing, and the resulting lighting contrast between the principal subject and the surrounding objects is often quite high. This often causes printer-classification failures. These failures, brought about by non-uniform illumination and over-exposure of the principal subject, induce a printer algorithm used in an photographic printer at a photofinisher to calculate improper printing conditions. As a result, the yield of acceptable prints is reduced and the photofinisher is compelled to reprint many negatives thereby adding time and cost to a photofinishing process.
In some cameras, the effects of such illumination problems are mitigated by providing means for selectively varying the flash illumination. For example, bounce flash, that is, directing or aiming the flash tube at a ceiling, white card or other light scattering source, is one solution that can provide a more uniform, diffuse illumination at short camera-to-subject distances that direct flash illumination can. However, power and size constraints have prevented its use in compact cameras. Flash units that permit bounce flash normally require increased capacitor size and, therefore, larger power sources. Also, such units usually have relatively long recycle times.
Another solution to the illumination problems encountered in compact cameras involves the use of a flash unit in which the direction of the light beam emitted from the flash can be varied relative to the optical axis of the objective lens of the camera. Redirecting the light beam from a flash unit can improve the picture quality by reducing the illumination of a close-up subject. U.S. Pat. No. 3,882,514 (issued to T. Graham on May 6, 1975 and hereinafter referred to as the '514 patent) describes a camera with a self-contained adjustable flash that varies the light beam direction from the flash as the objective lens is manually focused.
A further flash-illumination solution involves the use of a wide-angle flash reflector/diffuser. Wide-angle direct flash coverage usually provides improved scene illumination, particularly at short camera-to-subject distances, when compared to units that vary the direction of the flash beam as in the '514 patent. However, although wide-angle direct flash units have served the purpose, these units have not proved entirely satisfactory because they reduce flash range. As such, cameras have been developed with a moveable diffuser that provides wide-angle illumination only when manually engaged. Wide-angle illumination at short camera-to-subject distances is achieved, usually at short camera-to-subject distances, by manually moving the diffuser to a position in front of the flash. Narrow, more specular illumination at longer distances is achieved by retracting the diffuser away from the flash.
U.S. Pat. No. 4,176,930 (issued to T. Imura on Dec. 4, 1979) describes a camera with a self-contained flash system having a moveable diffuser for varying the illumination angle. The camera comprises two objective lenses; namely, a telephoto lens and a wide-angle lens, that may be manually selected by the operator. The wide-angle lens is selected by manually moving a bracket that moves the diffuser into the optical path of the flash for wide-angle illumination. When the telephoto lens is selected, the bracket movement retracts the diffuser away from the optical path of the flash to increase flash range and provide narrow, specular illumination.
A method for automatically varying the illumination angle in response to the selection of the focal length of the objective lens is shown in U.S. Pat. No. 4,851,868 (issued to N. Faniguchi et al on Jul. 25, 1989). Here, a flash unit is coupled to a microcomputer that receives digital data that represents the focal length of the lens. The focal length data is transmitted by the microcomputer to a control mechanism that alters the angle-of-illumination of the flash unit: such that this angle coincides with the angle-of-view of the objective lens.
As the preceding summary illustrates, none of the illustrative prior art embodiments set forth above varies flash illumination in a simple, inexpensive and automatic manner to prevent a subject, particularly at relatively short camera-to-subject distances, from becoming excessively illuminated during a flash exposure. Consequently, these embodiments which vary illumination angle will require a secondary means for regulating exposure, such as electronically quenching the flash emission or changing the size of the objective lens aperture. Use of such systems or associated mechanisms to provide this type of exposure control adds cost and complexity to the camera; as such, they are not suitable for use in inexpensive camera products.
Thus, for quite some time, a need has existed in the art for a simple, inexpensive and automatic system for varying flash illumination in a manner that effectively prevents a principal subject, particularly when located at relatively close camera-to-subject distances, from becoming over-exposed by the flash. Such a system, if it were to exist, would be well suited for use in inexpensive camera products, including fixed-focus, fixed-aperture cameras that do not include secondary exposure control systems or mechanisms.